1. Field of the Invention
This invention relates to a method for combustion of a solid carbonaceous material, in particular, solid fuels such as coal, municipal solids waste, biomass, refuse derived fuels, and the like in industrial and utility stoker boilers which are co-fired with other fuels such as gaseous, liquid and/or solid fuels. The method and apparatus of this invention provide a reduction in emissions, an increase in firing rate and possible improvements in efficiency, and a reduction in the amount of gaseous, liquid and/or solid fuel consumption for co-firing in a cost-effective manner in comparison with conventional solid fuel, waste, biomass and the like-fired industrial and utility stoker boilers.
2. Description of Prior Art
Most of the existing stoker processes and apparatuses for combustion of solid fuels, waste, biomass and the like include a combustion chamber equipped with a sloped or horizontal vibrating stoker grate that reciprocates or travels to move the fuel from the fuel inlet side of the combustor to the ash removal side of the combustor. A portion of the combustion air, generally equivalent to about 1.0 to about 1.3 of the fuel stoichiometric requirement is supplied under the stoker grate. Such combustion air is typically called undergrate air, and is distributed through the stoker grate to dry and burn the fuel present on the stoker grate. The fuel is first dried on the drying portion or drying grate of the stoker grate, then combusted on the combustion portion or combustion grate of the stoker grate. The residual fuel that primarily includes ash and carbon is then decarbonized or burned on the burnout portion or burnout grate of the stoker grate. The bottom ash is then removed through an ash pit. To assure carbon burnout, a high level of excess air, compared to the amount required for carbon burnout, is maintained at the burnout grate. In addition to other species, the products of fuel drying, combustion and burnout contain products of incomplete combustion such as carbon monoxide and total hydrocarbons, oxides of nitrogen, such as NO, NO2, N2O and other nitrogen-bearing compounds such as NH3, HCN and the like.
The majority of NOx evolved from the stoker grate, also referred to herein as the primary combustion zone, is believed to form from the oxidation of nitrogen-bearing compounds and a smaller portion formed from the oxidation of molecular nitrogen.
Additional air or overfire air is usually introduced above the stoker grate, referred to herein as the secondary combustion zone and mixed with the products evolved from the primary combustion zone to burn out the combustibles.
Nitrogen-bearing compounds that evolve from the fuel react with oxygen in and downstream of the secondary combustion zone, forming significant additional NOx. Because of the low combustion temperatures in and downstream of the overfire air injection, most of the NOx formed in this zone is by the oxidation of nitrogen-bearing compounds (less than about 10% are formed in this zone by the oxidation of molecular nitrogen).
In most cases, a boiler is an integral part of the combustor to recover the heat generated by the combustion of the solid combustible material. In some cases, cooled flue gases from downstream of the boiler are recirculated back into the primary and/or secondary combustion zone to reduce oxygen concentration and to lower combustion temperatures and, thus, are believed to enable some decrease in oxides of nitrogen formation. Disadvantages of flue gas recirculation include generally higher concentrations of products of incomplete combustion within the flue gases and within the stack gases due to reduced combustion efficiency, reduced boiler thermal efficiency, and increased capital and operating costs.
One approach to the reduction of NOx, CO, and total hydrocarbon emissions in industrial and utility boilers fired with solid fuel, waste, biomass and like-type fuels is the introduction of a fuel, such as natural gas, into the combustion products generated by the primary combustion zone.
U.S. Pat. No. 5,205,227 teaches a process and apparatus for combustion of a combustible material in which the combustible material is introduced onto a stoker grate in a combustion chamber and burned, forming a primary combustion zone. A fuel or fuel/carrier fluid mixture is supplied into the combustion chamber to create an oxygen deficient secondary combustion zone for NOx reduction and other nitrogen bearing compounds decomposition above the primary combustion zone. An oxidizing fluid is supplied into the combustion chamber above the oxygen deficient secondary combustion zone for thorough mixing with combustion products and at least partial burnout of combustibles in an oxidizing tertiary combustion zone.
A substantial amount of work has been directed to the disposal of solid waste material for the purpose of improving efficiency, reducing NOx emissions, more stable combustion and lower capital and operating costs. A substantial amount of work also has been conducted for these same reasons in connection with solid fuel fired industrial and utility boilers. See for example U.S. Pat. No. 5,957,063 to Koseki et al., which teaches a combustion system having a thermal decomposition section in which solid combustibles are thermally decomposed or partially burned so as to generate combustible gases and a combustion section in which the combustible gases are burned. The apparatus in accordance with one embodiment is a stoker-type boiler in which thermal decomposition of the solid fuel is initiated by a burner, such as a natural gas burner, disposed above the grate.
U.S. Pat. No. 5,823,122 to Chronowski et al., teaches a system for the gasification of solid biomass fuels and for combustion of the fuel gas produced therefrom comprising a gasification zone connected to a solid biomass fuel supply and to a gasification air supply, a predetermined ignition point, a pathway for conveying fuel gas from the gasification zone to the ignition point, and a combustion air injection device for mixing fuel gas and combustion air at the ignition point to initiate combustion of the fuel gas and the combustion air.
U.S. Pat. No. 5,657,705 to Martin et al., teaches a furnace for pyrolysis of solid waste material comprising a cylindrical cavity rotating around its lengthwise axis, a combustion chamber located around the cavity and injectors for introducing fuel and oxidant into the chamber.
U.S. Pat. No. 5,655,463 to Good teaches a furnace for decomposition of waste material comprising a decomposition chamber, a waste preheat chamber disposed above the decomposition chamber by which the waste material to be decomposed is preheated prior to entry into the decomposition chamber, and an afterburner chamber which operates under vacuum such that the gases and vapor from the preheat chamber and the decomposition chamber are drawn through the decomposing solid fuel.
U.S. Pat. No. 5,241,916 to Martin teaches a method for supplying combustion air during grate firings in which the primary combustion air is introduced into the fuel and secondary combustion air is introduced directly into the flow of exhaust gas and in which some of the exhaust gas is tapped off from the flow of exhaust gas and returned to the combustion process.
U.S. Pat. No. 4,848,249 to LePori et al., teaches a method and apparatus for the gasification of biomass in a fluidized bed gasifier in which the products of combustion of a fuel are passed through the distributor plate for preheating of the biomass.
Not with standing the improvements that have been made with respect to reducing pollutant emissions from utility and industrial boilers fired with solid fuel, waste, biomass and like-type fuels as exemplified by the aforementioned prior art, there remains a need for combustion processes and apparatuses which provide lower co-firing fuel consumption for the same emission reduction rate, potential for much higher emission reduction at the same co-firing fuel rate, more stable combustion, higher turndown ratio, increased firing rate and boiler thermal efficiency, and lower capital and operating costs.
Accordingly, it is one object of this invention to provide a method for combustion of a solid fuel and/or waste material which addresses the needs for further improvements as discussed hereinabove.
These and other objects of this invention are addressed by a method for combustion of a solid carbonaceous material, including solid fuels such as coal, waste, refuse derived fuels and biomass on a stoker grate in which the solid carbonaceous material is preheated and pyrolyzed in a pyrolysis zone of a stoker to form pyrolysis products, and partially devolatilized material is combusted on the stoker, forming a primary combustion zone. An oxidant is introduced into the combustion chamber downstream of, typically above, the primary combustion zone, forming a secondary combustion zone in which substantially complete combustion of the unburned products is carried out. As will be discussed hereinbelow, the solid carbonaceous material may be preheated prior to its introduction onto the stoker grate or it may be preheated after introduction into the charging zone. Introducing an overfire oxidant into the combustion chamber above the stoker grate or downstream of the primary combustion zone results in combustion of the unburned products. In accordance with a particularly preferred embodiment of this invention, preheating of the solid carbonaceous material is carried out on the stoker grate by the products of combustion from combustion of a co-fired gaseous, liquid or solid fuel which is continuously introduced directly into the solid carbonaceous material.
The method of this invention is carried out in an apparatus for combustion of a solid carbonaceous material comprising at least one wall enclosing a combustion chamber, a stoker grate disposed in a lower region of the combustion chamber, preheating means for preheating the solid carbonaceous material, and injection means for continuously introducing a pyrolysis agent into the solid carbonaceous material. In accordance with one particularly preferred embodiment of this invention, the injection means introduce the pyrolysis agent directly into the solid carbonaceous material disposed on the stoker grate.